libx264: Don't leave max_b_frames as -1 if the user didn't set it

max_b_frames is initialized to -1 for libx264, to allow
distinguishing between an explicit user set 0 and a default not
touched 0 (see bb73cda2).

If max_b_frames is left as -1, this affects dts generation (where
expressions like max_b_frames != 0 are used), so make sure it is
left at the default 0 after the libx264 init function returns.

This avoids unnecessarily producing dts != pts when using
profile=baseline.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/libx264.c

@@ -448,6 +448,9 @@ static av_cold int X264_init(AVCodecContext *avctx)
     // update AVCodecContext with x264 parameters
     avctx->has_b_frames = x4->params.i_bframe ?
         x4->params.i_bframe_pyramid ? 2 : 1 : 0;
+    if (avctx->max_b_frames < 0)
+        avctx->max_b_frames = 0;
+
     avctx->bit_rate = x4->params.rc.i_bitrate*1000;
 #if FF_API_X264_GLOBAL_OPTS
     avctx->crf = x4->params.rc.f_rf_constant;